Bearing systems for sealless pumps are used to radially support the rotor assembly used to drive the pump. Axial loads occurring during start-up, operation, or upset conditions are also carried by the bearing system. Under normal conditions, the rotor assembly is submerged in the fluid being pumped, and by design, such fluid is circulated around the rotor assembly and bearings to cool the motor or driver and lubricate the bearing system.
Fluids that are pumped by sealless pumps are generally corrosive, highly volatile, low viscosity, having very high or low temperature (ranging for example from cryogenic to 1,000.degree. F.), and oftentimes contain hard erosive contaminants. These conditions must be mitigated by pump design, or handled by the bearing system.
Generally speaking, special grades of carbon-graphite bushings or bearings operating against stainless steel provide the best performance over the widest range of conditions. However, for difficult applications or application with compatibility issues, special bearing materials may be necessary. For example, fluid compatibility may dictate the use of bearings made of Teflon or Rulon materials, while high temperature and particulate contamination may require the use of bearings made of hard bearing materials such as aluminum oxide (ALOX), tungsten carbide, or silicon carbide (SiC). These materials are characterized by high values of hardness (Vickers - 2000+), and in the case of ALOX and SiC, are extremely brittle and easy to fracture due to rough handling or vibration. Also, because hard bearings typically have relatively low thermal conductivity, they are very susceptible to heat build-up due to dry running or cavitation. These conditions, which can be common occurrences with sealless pumps, can result in bearing failures.
Failure modes of hard bearings in sealless pumps can be particularly catastrophic, since when hard bearings fail, they usually shatter like glass, making hundreds of razor sharp shards which can pierce stainless steel liners or shrouds exposing motor windings or external magnets to the pumping fluid. Besides requiring very expensive repairs, the resulting leakage of volatile or corrosive pumping fluids (chemicals) can be extremely hazardous.
There is thus a need for a bearing system for rotary fluid pumps including particularly sealless pumps which can advantageously use hard bearings while minimizing the dangers of bearing failure associated with their use.